"We've been able to identify and strike strategic targets that place increased pressure on their ability to recruit and sustain operations,” Air Force Lt. Gen. Charles Brown, who oversees the air war, said in a statement.

The Islamic State cut pay to its fighters by as much as half, after much of the terror group's cash literally went up in smoke, according to the Pentagon. Revenue from oil, the militants' principal source of income, has also taken a hit.

Such a strategy is not new. “Targeting an enemy’s economy is almost as old as air power,” said Richard Muller, a military history professor at the Air Force’s School of Advanced Air and Space studies at Maxwell Air Force Base in Montgomery, Ala. “This is an explicit return to something that hasn’t been at the forefront for many years.”

The Islamic State is viewed by the military as a hybrid terror organization that set itself apart from al-Qaeda and other groups by controlling large swaths of territory and governing people. That made the Islamic State, also known as ISIL or ISIS, vulnerable to air attacks since running an economy forces the organization to be more visible.

The coalition launched Operation Tidal Wave II last year, an air campaign targeting the Islamic State’s oil refineries, collection points and other facilities. It was named after the 1943 allied operation against oil refineries that fueled the Nazi war machine.

In the current campaign, military planners are making a similar effort to get at the oil revenue keeping the Islamic State afloat. Coalition pilots hit oil facilities in Iraq and Syria early in the campaign, but in November commanders began to systematically target tanker trucks, oil collection points, pump stations and wellheads.

The State Department estimated earlier this year that the Islamic State's oil revenue dropped to about $1 million a day from a peak of $1.3 million. Since then, the coalition continues to attack oil facilities almost daily.

Last October the coalition broadened its economic targets to include Islamic State cash distribution sites in Iraq and Syria, with most of the 15 strikes against the financial centers occurring since January, according to the military statistics.

The terror group conducts most of its business in cash, since its access to the banking system has been largely cut off. That means storing piles of cash in warehouses and vaults.

“We have a greater understanding of patterns of life, and we have strategically and precisely employed coalition air power to effectively target funding streams and other key targets wherever they may reside,” said Lt. Col. Chris Karns, a spokesman for Air Force Central Command.

Historically, attacks on economic targets were considered legitimate military objectives if tied directly to an enemy’s ability to wage war and if civilian casualties can be minimized.

During the 1999 air campaign in the Kosovo war, NATO aircraft attacked a number of economic targets, including factories belonging to business associates of former Yugoslav president Slobodan Milosevic, which helped pressure him to withdraw his forces from Kosovo.

The precision of today’s munitions and aircraft allows coalition commanders to minimize civilian casualties. Leaflets warning drivers to abandon their vehicles are dropped before the coalition targets tanker trucks.

Military planners also try to minimize destruction to a country’s economy. The Pentagon has said the attacks on the terror group’s oil infrastructure can disable wells and other facilities without causing permanent destruction.

“We’re not going to do irreparable damage to an economy,” said James Poss, a retired Air Force major general. “You’re going to have to deal with it after the war is over.”

The expansion of the air war to include economic targets is long overdue, some analysts say.

“From the very beginning we needed to treat the Islamic State as a state and not just an insurgency,” said David Deptula, a retired Air Force lieutenant general.